The present invention relates in general to numerical control systems, and in particular to contouring or path controls for moving a member simultaneously along two or more axes according to program of command signals. Such numerical control systems are per se well known in the art; they find use in controlling multi-axis machine tools but are equally well applied to other devices such as drafting machines, welding or flame cutting machines, painting or assembly robots, and so on.
The invention pertains more specifically to improvements in such numerical contouring control systems which are randomly interruptable, for emergency purposes or otherwise, to bring the controlled member to an immediate stop at a random point on the programmed path--this sometimes being called the "feedhold" function. When the member is so stopped, it is often moved away from its stopping point in one or several manually controlled motions so that (in the machine tool environment) a cutting tool may be inspected or changed, or the workpiece may be measured or inspected. When the controlled member is so removed from the programmed path--in directions and distances which cannot be determined or forecast in advance--it is vital that the member be returned to its stopping point on the path before the system is restarted into normal operation. Otherwise, the member might be driven into damaging contact with the workpiece, or the subsequent motions of the member may never get back to precise following of the desired path.
In some prior numerical control systems, a feedhold stop, followed by arbitrary movement of the controlled member away from the path stopping point, required that the punched tape or other source of block command signals be "backed up" to a known block in the program sequence, and that the member be moved by the operator to reference coordinates specified by the programmer in a data sheet supplied to the operator. This was inconvenient and time consuming, and could result in damage to machine, cutter, or workpiece if the operator forgot to re-reference the system before hitting the restart switch. In other systems, a feedhold signal would let movement of the member progress until a specified location along the path was reached--so that stopping of the member might be delayed until after some damage occurred. If such damage did not occur, and the member was moved away from the stopping point on the path, the machine operator's forgetting to restore the member to a reference location before actuating the restart switch could produce the same difficulties noted above.